The invention relates generally to electrical circuits for operating arc discharge flashlamps, and more particularly, to a more efficient circuit for operating a plurality of flashlamps.
Such flashlamps are employed in a variety of applications; for example, flash photography; reprographic machines; laser excitation; and warning or directional flashers for airplanes, towers, road barricades, marine equipment, and tower-mounted approach lighting systems for airport runways. An indicator structure application in which the DC powered circuit of the present invention is particularly useful is described in co-pending application Ser. No. 937,649, filed concurrently herewith and assigned to the present assignee.
Flashlamps of the type referred to herein generally comprise two spaced-apart electrodes within an hermetically sealed glass envelope having a rare gas fill, typically xenon, at a sub-atmospheric pressure. In typical prior art operating circuits, such lamps are connected across an energy storage device, such as one or more capacitors, charged to a substantial potential, but insufficient to ionize the xenon gas fill. Upon application of an additional pulse of sufficient voltage, the xenon is ionized and an electric arc is formed between the two electrodes, discharging the storage device through the flashlamp, which emits a burst of intense light. In many cases, the pulse voltage is applied between an external electrode, such as a wire wrapped around the envelope, and one of the electrodes; this is referred to as shunt triggering. However, in other cases, an external wire is not feasible since it may result in an undesirable arcing between the trigger wire and a proximate lamp reflector, or else the high potential applied to the external trigger wire might be hazardous to operating personnel. In those cases, the lamp may be internally triggered by applying the pulse voltage directly across the lamp electrodes, a technique referred to as injection triggering. Usually, the voltage required is about 30% to 50% higher than that required to trigger the same lamp with an external trigger wire, and the trigger transformer secondary must carry the full lamp circuit.
In applications requiring two (or more) flashlamps, the lamps have been series-connected across the storage capacitor means, with a single injection trigger circuit used for the series lamp combination. Where two lamps are required to be flashed in alternate sequence, a separate storage capacitor and RC trigger supply has been employed for each lamp, with a gating signal alternately activating the trigger circuits. Accordingly, such prior art circuits for sequenced flashlamps add significantly to the cost and bulk of the power supply.
One approach for overcoming the aforementioned shortcomings of conventional flashlamp arrangements is described in co-pending application Ser. No. 865,405, filed Dec. 29, 1977, now abandoned and assigned to the present assignee. Briefly, the operating circuit of this co-pending application employs a single storage capacitor means and uses the charging current of the storage capacitor, as well as the discharge current, for purposes of lamp energization. More specifically, first and second arc discharge flashlamps are series connected across a supply voltage source comprising a large direct current storage bank. The storage capacitor means is connected between the junction of the lamps and one terminal of the source. Respective injection or shunt means are provided for coupling trigger pulses to each lamp, and a succession of high voltage trigger pulses are alternately applied through the respective coupling means to the lamps. Each trigger pulse applied to the first lamp effects an arc path therethrough for charging the capacitor, and each trigger pulse applied to the second lamp effects an arc bath therethrough for discharging the capacitor. Hence, the storage capacitor is charged through one lamp and discharged through the other in response to trigger pulses, which are applied in alternate sequence to the lamps. In essence, the lamps function as alternately actuated switches for charging and discharging the capacitor.
Although offering a number of significant advantages, the above-discussed circuit also has a disadvantage in that the power source requires a large DC storage means, such as a bank of capacitors. This tends to add to the bulk, weight, and expense of the DC power source. Such factors detract from efforts to provide compact, low-cost flashlamps for photographic applications, lightweight runway flashers for mounting on frangible towers, or various other indicator structures such as road barricades.
One approach which has been taken to overcome such disadvantages, with respect to the discharge storage bank, is described in co-pending application Ser. No. 865,564 filed Dec. 29, 1977, now U.S. Pat. No. 4,142,130, and assigned to the present assignee. This improved circuit uses the above-described multiflash arrangement wherein a pair of flashlamps are alternately triggered to charge and discharge a storage capacitor through the lamps, but in this instance, the lamps and storage capacitor means are connected directly to an AC source. In this manner, the first lamp draws the major portion of its operating voltage directly from the AC source with no substantial energy storage means located therebetween other than a storage capacitor means. Of course, this improved circuit has the drawback of requiring the availability of an AC power line, and thus is not suitable for applications in remote locations where only battery sources may be employed. Further, the circuitry continues to require a separate RC timing circuit for controlling the triggering of each flashlamp.